Preheat the oven to 350°F (175°C). Butter an 8-inch square baking pan and line it with parchment paper, allowing a 2-inch overhang on two sides. Lightly butter the parchment lining (not the overhang).

Whisk together the flour, baking powder, and salt in a medium bowl. Set aside.

In the bowl of an electric mixer fitted with the paddle attachment, beat the butter and brown sugar on medium speed until pale and fluffy, about 3 minutes. Add the eggs and vanilla, and mix until fully combined.

Reduce the mixer speed to low and gradually add the flour mixture. Mix until just combined, scraping down the sides of the bowl as needed. Gently fold in the butterscotch chips, cashews, and toffee bits.

Pour the batter into the prepared pan and spread it evenly with a rubber spatula. Sprinkle the top with flaky sea salt.

Bake for 40-45 minutes, or until the top is golden brown and a cake tester inserted into the center (avoiding the edges) comes out with a few moist crumbs but no wet batter.

Let the blondies cool in the pan for 15 minutes, then lift them out using the parchment overhang. Transfer to a wire rack to cool completely before cutting into 9 large or 16 small squares.

Store in an airtight container at room temperature for up to 3 days.